Project Summary ? Abstract The perception of odors begins in the olfactory epithelium when odorant ligands bind on olfactory sensory neurons, each of which expresses only 1 of ~1200 candidate receptors. Axons coming from neurons expressing the same odorant receptor converge into only 2/3 glomeruli/olfactory bulb where they synapse onto projection neurons, each of which innervates a single glomerulus. Deep to the glomeruli, within the external plexiform layer (EPL), odor coding is tuned by local synaptic circuits. Deciphering connectivity with the EPL is the first step toward understanding the mechanisms of central odor processing. Several hypotheses of local processing within the EPL have been proposed and its synaptic organization is often presented as canonical. However, we lack a fundamental understanding of the how the diversity in interneuron, granule cell (GC), structural, molecular, and topographical organization affects EPL local circuits, which is an impediment to interpreting functional studies. Here we propose a series of testable hypotheses and experiments on factors that may influence the organization of GCs and their local synaptic circuits. These studies build on our prior developmental and ultrastructural work as well as a systematic review of the current literature and the recognition that fundamental features of EPL organization and connectivity have been presumed, but not empirically tested. First, we propose to address the hypotheses that the clonal history, timing and order of GC neurogenesis are determinants of their organization/distribution in the olfactory bulb. Using multiple strategies to track neurogenesis, cell lineage/fate we will assess the spiny dendritic arbors of GCs beginning in the embryo and continuing at regular intervals to those generated up 200 days of age. In addition, and not among prior studies, we propose to carefully assess the organization of the GC basal dendrites which are the primary recipients of incoming centrifugal modulation (Rothermei and Wachowiak, 2014; Kapoor et al., 2016; de Almeida et al., 2015). Second, we propose to test the hypothesis that the synaptology and molecular features of the dendrodendritic synapses in the EPL vary as a function of age. Presently, little is known of the structural features that regulate dendrodendritic synapses or how the molecular properties of the mitral to GC and the reciprocal GC to mitral cell synapses may differ. The analyses will address that fundamental problem and provide a sound foundation for the interpretation of functional analyses of odor processing. In addition, we will address the spatial distribution of synaptic appositions along 2o dendrites and the degree to which the features of the synaptic specialization change with age. Because the questions we propose to address are important throughout the nervous system, we anticipate that the results will have broad implications for understanding targeting, laminar specificity, and synaptic connectivity of neurons throughout the brain.